It is known in the art to spin an object to be coated, during the coating process, such that centrifugal force aids in the even distribution of the coating material, and further such that the effects of any irregularities in the application process are negated by the fact that the object is spun past such irregularities, thereby averaging the effect over a substantial area of the surface of the object. Spin coating has proved beneficial in a great variety of applications wherein a very thin, very uniform coating such as of a resin or application of fluids such as developing fluid or cleaning agent is required. Among the applications are the coating of a magnetic coating on magnetic recording disks, and the coating of photo-resist fluid, and the like, during the manufacture of semiconductor products. The semiconductor products on which the process is used may range in size from relatively small wafers for the manufacture of integrated circuits to very large "flat panel display screens. Despite the many inherent advantages of spin coating, there have remained a variety of problems associated with this process, for which numerous inventions have been devised to cure one or more.
U.S. Pat. No. 5,069,156 issued to Suzuki, addresses the problem that substrates which are other than circular in cross section create an air turbulence when they are spun at high speed, thus causing uneven coating of the substrate. Suzuki teaches that providing a "spin cup" which surrounds the substrate and spins in unison, therewith, causes air surrounding the substrate to be rotated generally along with the substrate and the spin cup, thus reducing any turbulence.
The problems caused by air turbulence around the spinning substrate are further addressed by two different means in the teachings of U.S. Pat. No. 4,889,069 issued to Kawakami, and in those of U.S. Pat. No. 5,013,586 issued to Cavazza. Kawakami provides a cover over the spinning substrate and a vacuum apparatus for drawing air past the substrate as it spins, thus providing for a generally even air flow about the substrate during the coating process. Cavazza addresses the problem by providing a cover which spins in unison with the substrate and a spin cup, thus causing the substrate to spin in, essentially, still air, since the entire enclosure within which the substrate is housed is spinning in unison.
It is also known in the art to minimize the effect of non-circular substrate by providing a circular "chuck" with a recessed surface into which the substrate fits, such that the exterior presented to the surrounding air is the circular form of the chuck. This solution helps to solve the problem of air turbulence while presenting additional problems such as that the chuck must be cleaned after each substrate is treated because the areas of the chuck surrounding the substrate become contaminated with the coating fluid.
Yet another innovation in the field was provided by U.S. Pat. No. 3,730,760 issued to Machmiller. The Machmiller invention addressed a problem caused by the fact that centrifugal force is much greater toward the edges of a spinning object than toward the center. This produces the undesirable result that a coating is thinned considerably near the edges while it remains thicker near the center. Machmiller solved this problem by causing the substrate to spin vertically, rather than horizontally, such that gravity works in conjunction with the centrifugal force to assist in providing a more even coating. This "vertical spin coating" is now widely used in the field, although the older "horizontal spin coating" is also still widely in use because vertical spin coating, itself, suffers from several inherent problems. Primary among the problems associated with vertical spin coating is that coating fluid which is spun from the substrate may strike the surrounding portions of the spin coating apparatus and then drip back down upon the substrate. This problem is not found exclusively in vertical spin coating, as drops of fluid from a horizontally spun substrate may also cause a similar problem, particularly where it is required to spin the substrate at high speed such that fluid drops are flung about within the coating enclosure at high speed. However, the problem is certainly greater in vertical spin coating applications, since drops of fluid are there flung from the substrate to strike portions of the enclosure which are directly over the substrate. Another problem is that it is frequently more difficult to load and unload substrate from a vertical spin apparatus.
Yet more innovations have been introduced to enhance the spin coating process. For example, it is known to provide a cover over the spin coating drum which filters the ambient air as it enters the drum, such that the environment within the spin coating drum is even more free of pollutants than is the air in the "clean" room in which the spin coating apparatus is operated. It is also known in the prior art to introduce a shield or "baffle" in front of the object to be treated in order to reduce turbulence around it.
All of the above prior art innovations have made significant contributions to the field. However, to the inventor's knowledge, no prior art innovation has provided a means to prevent dripping from the apparatus onto the substrate during vertical spin coating. Furthermore, each of the prior art means for preventing turbulence has provided its advantage at the expense of other desirable aspects, such as processing speed, control of the process, and/or cost. No prior art spin coating means, to the inventor's knowledge, has reduced or controlled the turbulence of air around spinning non-circular or circular substrates without sacrificing processing speed and/or without the necessity for a separate exhaust fan and associated controls.